docs: def _compute_raw_image_norm(self, data):
          """
          Helper function that computes the uncorrected inverse normalization
          factor of input image data. This quantity is computed as the
          *sum of all pixel values*.
  
          .. note::
              This function is intended to be overriden in a subclass if one
              desires to change the way the normalization factor is computed.
  
          """
          return np.sum(self._data, dtype=np.float64)